Yes it supports the ISO vertical Enter, but not the XT’s specific vertical Enter – Google Model M ISO layout for example.  Then you can put another key in the barrel that it frees up.

I guess the real question is: will anything in the firmware prevent an arbitrary barrel registering as an individual key – since the “extra” key freed up would normally be covered up in either ISO or ANSI. (I know this seems odd, and it’s a very specific use case; the goal is to better support a semi-ergonomic split keyboard layout for the hands’ home positions.)

I think the answer you’re giving is…yes?

There are no issues with that uncovered part of the key. You can use the xwhatsit GUI to map it to a key of your choice or instruct the keyboard to ignore the key press through the GUI. To learn more about xwhatsit’s controller (the likely controller being used with this project, but not 100% the choice just yet) please see the Code tab of this site.